The determination of the solids content in drilling fluids (drilling "muds") is important in the drilling for hydrocarbons. Properties of drilling fluids such as density, viscosity, gel strength, and filtration rate depend to a considerable extent on the solids content of the drilling fluid. As the solids content increases, the drilling fluid density (or "mud weight") also increases, and mud weight has a direct effect on drilling rates. The viscosity of drilling fluids is affected by the relative quantity of solids present, and knowledge of this quantity may afford an explanation of certain undesirable properties and indicate the treatment to be used. For example, if the solids content of a thick water-base drilling fluid is excessive, water instead of chemicals should be used to thin the drilling fluid. Improper solids control may lead to an unnecessarily expensive drilling program and, in some instances, could even jeopardize the completion of drilling operations. Generally, there are two sources of solids in drilling fluids: (1) solids which are present from drilling fluid additives, or "solids-additives", and (2) solids present from the formation itself or "formation solids". It is important to accurately determine the amounts of solids, both from drilling fluid additives and the formation, in the drilling fluid. With accurate solids assessment, efficiency in a solids control circulation system can be achieved, and problems associated with drilling fluids can be diagnosed.
Two of the most common methods for determining the solids content of a drilling fluid sample are the retort analysis method and the cation exchange capacity (CEC) method. Both of these methods are well established in the art. Briefly, the retort analysis method determines total solids content in a drilling fluid by heating a fluid sample with known weight and volume to high temperatures to evaporate the liquid portions of the sample; what remains is the solids content. This test however does not provide information to separately determine the formation solids content and solids-additives content. See for example, American Petroleum Institute (API) Publication RP13B, pages 13-15, for a brief summary of the retort analysis method. The CEC method, on the other hand, correlates the number of exchangeable cations in the sample (cations in the sample that can be replaced by another cation, such as barium or ammonium) to the low gravity solids content. See for example, Van Olphen, H., "An Introduction to Clay Colloid Chemistry", Wiley-Interscience, New York, 1977, Chapter 5, and American Petroleum Institute Publication RP13B, "Recommended Practice for Standard Procedure in Testing Drilling Fluids", for a brief summary of CEC methods used in determining solids content in drilling fluid. The CEC method requires significant care in running the test and is not particularly suitable for wellsite use. In addition, the CEC does not provide a measurement of the formation solids and of the solids-additive, separately.
U.S. Ser. No. 175,081 to Kroeger et al. discloses a method for determining at wellsites the swelling-clay content of shales and shaly sandstone earth formations by dielectric measurements. Kroeger et al.'s method includes washing a sample of an earth formation with a fluid having a water activity substantially less than that of water to which a fluid cation may be added, measuring the sample's dielectric constant at a preselected frequency, and comparing the results of this measurement to calibration curves to obtain a measurement of the swelling-clay content of the formation. Kroeger et al.'s method describes different levels of determinations depending on the nature of the formation samples.
Currently, dielectric measurements are utilized for other, unrelated purposes. For example, dielectric measurements are utilized in logging tools for making determinations of the water and hydrocarbon content in sandstones and carbonates. These logging tools are not designed for making determinations of the solids content in drilling fluids. In addition, these logging tools lose their effectiveness in high-salinity fluids.
To the best of Applicants' knowledge, dielectric measurements are not used for making determinations of solids content in drilling fluids. In fact, excluding U.S. Ser. No. 175,081 to Kroeger et al., prior art actually dismisses dielectric responses observed between 1-50 MHz in dilute aqueous solids suspensions as anomalies which vanish with increasing salinity. See for example, Raythatha, R. and Sen, P. N., "Dielectric Properties of Clay Suspensions in the MHz to GHz Range", Journal of colloid and Interface Science, February 1986, Vol. 109, No. 2, in general, and particularly see pages 305 and 308 wherein it is stated that the electrochemical effects (of solids) become unimportant at high salinities and the geometrical effects dominate.
There exists a need for a rapid and reliable wellsite method for the determination of the solids content in drilling fluids. It is desirable to be able to obtain, at the wellsite, timely estimates of the total solids content of drilling fluids, including the portion corresponding to solids-additives and the portion corresponding to formation solids.